Application availability is a measure used to evaluate whether an application is functioning properly and usable to meet the requirements of an individual or business. Application availability is determined based on application-specific key performance indicators (KPIs) such as overall or timed application uptime and downtime, number of completed transactions, responsiveness, reliability, and other relevant metrics. Real or perceived application failures are also taken into account, such as consistent errors, timeouts, missing resources, and DNS lookup errors.
To ensure acceptable service for users and reliable support for the business, organizations typically seek to maintain high availability. A high availability system is able to maintain continuous operations with an extremely low error rate for an extended period of time. To achieve demanding standards of high availability, such as the elusive “five 9s” (99.999 percent), a system must be designed from the ground up to ensure effective backup and failover for processing, data storage, and networking.
Server load balancing plays a critical role in ensuring high availability by enabling a rapid response to a server failure. A load balancer can be deployed as the front end to a cluster of servers, routing each incoming client request to a member of the cluster, and relaying the response back to the client. To ensure high availability and optimal service, the load balancer performs continual health checks of each server in the cluster, using probes to determine its eligibility for requests.
If a server becomes unavailable or falls below acceptable performance metrics, the load balancer detects the outage, stops sending requests to it, and redistributes traffic across the remaining members of the cluster. High availability load balancing further aids application availability by ensuring that the load balancer itself remains available.
Global server load balancing (GSLB) performs a similar function to server load balancing on a broader scale. GSLB provides load balancing, site failover, and web traffic management across multiple data centers and/or clouds (private cloud or public cloud). This makes it possible to ensure application availability based on factors such as the health, availability, and loading for each data center; client geographical locations based on their ISP DNS server address; and bandwidth utilization.
The A10 Networks Thunder® Application Delivery Controller (ADC) ensures high availability and rapid failover through load balancing, global server load balancing, and continuous server health monitoring.
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